Here (Las Vegas) as an example for last night.
Patriots were giving up 3.5 points.
If the Pats won by three and you chose them, you would lose.
If the Pats won by 4 or more, you would win $1 for every $1.10 that you bet
The over/under was 53.
The people that bet the under were paid as above.
Those that bet the over, lost.
Standard payout is $1/$1.10 taking into account the point spread (for FB and BB).
Over/Unders do not have a point spread, but the payout may vary slightly.
The line may also adjust the payout along with the spread.
In LV we are able to bet the first half, second half as well as the comlpete game for NFL.
when the NBA season has settled down, the same will apply.
NCAA at times will have totals and halfs depending on the game.